The present invention relates to a ceramic porous membrane bound by a titania, a ceramic filter using the same, and a method of manufacturing the same.
In comparison with polymeric membranes, etc., a ceramic filter in which a porous membrane having pores of a smaller diameter is formed on the surface of a ceramic porous substrate, is useful as a filter, etc. for a solid-liquid separation in view of a high reliability due to its excellent physical properties and durability, in view of a very small deterioration even after cleaning by acids or alkalis due to its high corrosion resistance and, further, in view of being capable of precisely controlling a pore diameter upon which filtering ability depends.
In the ceramic filter described above, performance of a ceramic porous membrane upon which filtering ability depends and a manufacturing method thereof are technical points when manufacturing the ceramic filter.
In general, a ceramic porous membrane can be obtained by a process in which a slurry comprising ceramic particles is deposited on the surface of a substrate, and then fired at a high temperature of not less than 1,300.degree. C. to thereby solid-phase sinter the ceramic particles with each other.
Further, Japanese Patent Application Laid-Open No. 63-274407 discloses a method of manufacturing a ceramic porous membrane in which a slurry comprising ceramic particles constituting aggregate to which an inorganic sol such as alumina sol or silica sol is added, is deposited on the surface of a substrate before firing (so-called a raw substrate), and then co-fired at 1,200.degree. C.
The former process can provide a porous membrane having an excellent strength and corrosion resistance owing to a strong binding by solid-phase sintering of the ceramic particles with each other. However, if firing is not effected at a high temperature of 1,300.degree. C. or more, a porous membrane having a practicable strength can not be obtained.
Firing at such high temperatures requires a large amount of energy. A setter, etc. on which porous substrates are disposed, must be prepared by a special equipment requiring a high cost made by, for example, a refractory body such as a silicone carbide. In addition, heat resistance of not less than 1,300.degree. C. is required also for the substrates. Accordingly, the material for the substrates unavoidably becomes expensive, resulting in increased product costs.
On the other hand, in the latter process, since firing takes place after forming a membrane by depositing a slurry on a raw substrate, ceramic sol is added to the raw substrate for ensuring handling strength. Further, ceramic sol is also added to a slurry constituting a membrane portion in order to synchronize a firing behavior of the substrate portion with that of the membrane portion.
Even in this method, although the temperature is somewhat lower in comparison with the former process, since firing at a high temperature of not less than 1,200.degree. C., there is a problem accompanied by the firing at a high temperature.
Further, in the porous membrane prepared by the latter process, a portion of fine particles derived from inorganic sol is more sensitive against corrosion in comparison with aggregate particles. Accordingly, in the case where a silica sol is used, there is a problem because it is low in alkali resistance.
On the other hand, in the case where an alumina sol is used, if a sufficient firing is not effected at a high temperature of not less than 1,200.degree. C., there is a problem as it is low in alkali resistance because .gamma.-alumina is formed. Accordingly, a firing at a high temperature becomes essential.